Generally, each pixel consists of a photosensitive element and an electronic circuit comprising, for example, switches, capacitors and resistors, downstream of which an actuator is placed. The assembly formed by the photosensitive element and the electronic circuit allows electrical charge to be generated and collected. The electronic circuit generally allows the charge collected in each pixel to be reset after a charge transfer. The role of the actuator is to transfer the charge collected by the circuit to a read bus. This transfer is carried out when the actuator receives the instruction to do so. The output of the actuator corresponds to the output of the pixel.
Thus, a detector comprises an array of similar pixels, each column (or each row) of adjacent pixels generally being connected to the same read bus.
In this type of detector, a pixel operates in two phases: an image capture phase, during which the electronic circuit of the pixel accumulates electrical charge generated by the photosensitive element, and a read phase, during which the collected charge is transferred to the read bus, by virtue of the actuator.
During the image capture phase, the actuator is passive and the electrical charge collected will change the potential at a connection point between the photosensitive element and the actuator. This connection point is called the charge collection node of the pixel. During the read phase the actuator is active in order to free the charge accumulated in the photosensitive dot, in order to transfer or copy it, or even copy the potential of the charge collection node to a read circuit of the detector.
The expression “passive actuator” is understood to mean that the actuator does not make electrical contact with the read circuit. Thus, when the actuator is passive, the charge collected in the pixel is neither transferred nor copied to the read circuit.
An actuator may be a switch controlled by a clock signal (it is generally a transistor). It may also be a follower circuit or any other device allowing the charge collected in the pixel to be communicated or transferred to the read circuit, for example it may be a capacitive transimpedance amplifier (CTIA).
This type of radiation detector may be used for imaging ionizing radiation, and notably X-rays or γ-rays, in the medical field or in nondestructive testing in the industrial field, or to detect radiological images. The photosensitive elements allow electromagnetic radiation in the visible or near visible range to be detected. These elements are not, or not very, sensitive to the radiation incident on the detector. Thus, a radiation converter called a scintillator is used to convert the incident radiation, for example an X-ray, into radiation in a wavelength range to which the photosensitive elements present in the pixels are sensitive.
During the image capture phase, the electromagnetic radiation, in the form of photons received by each photosensitive element, is converted into electrical charge (electron/hole pairs), and each pixel generally comprises a capacitor allowing this charge to be accumulated so as to change the voltage of the collection node of the pixel. This capacitor may be intrinsic to the photosensitive element, a parasitic capacitor then being spoken of, or added in the form of a capacitor connected in parallel to the photosensitive element.
Thus, according to the prior art, each pixel comprises one photosensitive region, comprising a single photosensitive element.
Current photosensitive elements cannot be directly adjusted to match variations in the flux of radiation. In the human eye, this adjustment is carried out by the iris, which tends to reduce the incident luminous flux under strong illumination. Likewise in a camera this function is achieved by way of a shutter. In a radiation detector, such as described above, this adjustment is much more difficult to achieve.
It has been sought to match variations in flux by adding a capacitor to each pixel, which capacitor may, if required, be connected in parallel to the photosensitive element. More precisely, in the case of low luminosity, the additional capacitors are disconnected in all the pixels of the detector. In the case where the detector is strongly illuminated, the capacitors of all the pixels are connected in order to reduce the voltage of the pixel. In other words, this capacitor allows the gain of the pixel to be modified via its transfer function between the number of photons received and the voltage of the pixel. The additional capacitor is connected by means of an electronic switch, such as, for example, a metal oxide semiconductor (MOS) transistor.
This solution, enabling use of the pixel in different gain ranges, has a number of drawbacks. Firstly the switch enabling connection of the additional capacitor interferes with the voltage of the node of the pixel, because it generates a leakage current. This current degrades the performance of the pixel, notably when the capacitor must be disconnected. Secondly, some of the area of the pixel is occupied by the additional capacitor, to the detriment of the area of the photosensitive element.